This invention relates to acoustical flowmeter systems and is particularly directed to an improvement in the acoustical flowmeters of the type described and claimed in the U.S. Pat. No. 4,003,252 entitled "Acoustical Wave Flowmeter" by E. J. DeWath which issued Jan. 18, 1977 and the flowmeter system of the type described and claimed in the U.S. Pat. No. 4,164,865 entitled "Acoustical Wave Flowmeter" by L. G. Hall and R. S. Loveland which issued August 21, 1979.
The invention of DeWath was directed to a flow meter having an unobstructed tubular wall thereby eliminating all impediments to the flow path of the fluid and eliminating all cavities in which debris might collect. The advantages of such a configuration is fully set forth in the DeWath Patent. To measure flow of a selected fluid in the DeWath flowmeter, however, required a calibration for that particular fluid and required a recalibration if the flow of a different fluid was to be measured since the flowmeter was not responsive to changes in fluid species or densities.
The Hall and Loveland invention improved the DeWath flowmeter by providing a flowmeter that measured flow accurately regardless of changes in fluid composition or temperature and by providing a flowmeter with a means for determining a change in velocity of sound of the fluid being measured.
In order to accomplish this, the Hall and Loveland acoustical wave flowmeter system had two spaced apart crystal transducers in the wall of the flowmeter conduit (sometimes called a cavity) to produce ultrasonic acoustic compressions at selected frequencies in the fluid within the cavity. The transducers were alternately switched into a transmit and a receive mode to generate upstream and downstream transmitted and received signals with an automatic means to adjust the transmitted frequencies to compensate for changes in velocity of the acoustic compressions in the fluid caused by changes in fluid composition and temperature. The electronic circuitry involved in the Hall and Loveland flowmeter system include means for measuring and storing signals representing the phase difference between the transmitting transducer signal producing the acoustic compressions and the signal produced by the receiving transducer during each of two successive transmit/receive cycles. Circuit means were provided to determine the difference between the signals representing the two successive phase differences wherein the sign of the difference corresponds to the direction of the fluid flow and the magnitude of the difference corresponds to the rate of fluid flow through the flowmeter. Circuit means were also provided to add the two successive phase difference signals together to obtain a signal proportional to the velocity of sound in the fluid moving through the flowmeter. This latter signal indicated the change in composition of the fluid flowing through the meter.
The Hall and Loveland flowmeter system had a phase lock loop in the receiver/transmitter system which included a phase adjustor comprising two operational amplifiers (8043C) and associated components. This phase adjuster was used during initial calibration to align the transmit and received signals to a 90.degree. relative phase difference. Without this phase shifter, this coincidence of signals would not be achievable.
The problem, however, is that as the frequency of the transmitted signals changed, the phase adjuster also changed its output which represented an error. The net result is that the circuitry made an adjustment for this error as if the velocity of sound through the fluid medium was changing. In other words, the VCO would automatically adjust the frequency to make up for the phase error produced by the phase adjusting circuit. Accordingly, it can be seen that there was a need for a phase shifter which was insensitive to frequency changes.
Another problem with the circuitry of the patented system was that the only way the system could be calibrated was to provide a fluid flow of a known fluid at a known flow level through the transducer, measure the result, and calibrate the system. There exists a need to calibrate the system without an external flow source and this invention accomplishes this need.
Accordingly an object of this invention is to provide a phase shifter which is insensitive to frequency changes in the system and to provide a self calibrator by which the system can be calibrated without the need of an external fluid flow source.